A tool head of this type is known (WO91/03345), in which those surfaces of the substrate plates exhibiting the capacitive measuring structures are disposed in centrifugally neutral arrangement, in the direct proximity of the rotational axis, in planes running perpendicular to the rotational axis. The first substrate plate is herein flatly connected, by its surface opposing the measuring structure and pointing axially outwards, to a bearing face of the slide, whilst the second substrate plate is stuck, by its surface exhibiting the measuring structure, against two mutually spaced mounting strips, the measuring structure being left free, which mounting strips are fastened, by their free ends projecting over the edge of the substrate plate, to an axially aligned mounting face of the main body, whilst the substrate plate engages in a central recess in this mounting plate. The effect of this fastening method is that that surface of the substrate plate which bears the measuring structure lies exactly flush with the mounting face of the main body. This arrangement is only however possible if the substrate plates are disposed in centrifugally neutral arrangement. There are however tool designs in which, because of central built-in elements, for example, a centrifugally neutral arrangement of the substrate plates is not possible. In the case of an eccentric arrangement of the substrate plates, the known measures, because of the centrifugal forces acting upon the substrate plates, cannot be adopted. Added to this is the fact that, especially in the use of ceramic substrate plates, measurement tolerances of .+-.100 .mu.m in the wall thickness can occur, which, in the previously known arrangement, cannot be readily compensated, considering that the gap width between the active surfaces of the substrate plates, in capacitive measuring structures, must necessarily measure only 10 to 20 .mu.m.